1. Field of the Invention
The invention relates to catalytically active platinum metal layers.
2. Description of the Related Art
Platinum metals is the generic term for the metals of subgroup VIII of the periodic table of the elements and comprises: Ru, Rh, Pd, Os, Ir and Pt. These metals are used in various chemical processes due to their catalytic properties. Examples of processes utilizing platinum metal catalysts are hydrogenation processes, e.g. in oil and grease production and chemical synthesis, like the Fischer-Tropsch reaction, or in environmental protection, e.g. car catalysts or water cleaning. Another example of the use of platinum metal catalysts is in plating processes where they serve as a seed layer for the deposition of metal ions on dielectric or metallic surfaces.
The catalytic activity of a platinum metal seed layer depends on the nature of the surface of the platinum metal layer, i.e., the smaller the seeds and the greater their number, the better is the catalytic activity. This is true of all of the platinum metal catalyst applications cited above.
Generally, a platinum metal is conventionally deposited onto a surface from an acidic solution containing a platinum metal salt. This standard procedure is known to result in unsatisfactory platinum metal layers having undesirable irregularities and inhomogeneous thicknesses. Also, the catalytic activity of these conventionally deposited platinum metal layers has been found to be insufficient for various applications.
Several attempts to overcome the above problems are known in the prior art. One of these approaches is described in U.S. Pat. No. 4,764,401 which relates to the application of platinum metals as catalyst layers for subsequent metal deposition in plating processes. Surfaces are activated therein using organometallic complexes of elements of the groups IB and VIII of the Periodic System whose organic moiety has at least one functional group which is suitable for fixing the activator to the substrate surface. Thereby a firmly adherent metal coating is achieved, but the catalytic activity is still unsatisfactory.
Another approach has involved the addition of surfactants. A rich variety of compositions have been suggested as surfactants. For example, EP-A-0,144,612 cites several surfactants added to a colloidal solution for activating surfaces for subsequent metallization. The adhesion of copper to a substrate surface was increased thereby, but colloidal solutions are being replaced by ionogenic systems nowadays.
Yet another approach is desclosed in IBM TDB, 08-81, p. 1525. Here, sodium lignin sulfonate is added, to a colloidal system, in order to obtain palladium films of more homogeneous thicknesses and with a greater number of palladium nuclei from an acidic palladium chloride solution.
Although the above approaches have served to ameliorate the problems associated with conventionally deposited platinum metal layers, the catalytic activity thereby attained is still insufficient for many applications and these approaches also require great quantities of very expensive platinum metals. As far as plating processes are concerned, only colloidal systems are discussed, and no efficient additive for modern ionogenic activation solutions is mentioned or described.